1. Field of the Invention
The present invention relates to a process for producing an ether compound. More particularly, the present invention relates to a process for producing an ether compound, for example, methyl-tert-butyl ether, by reacting a lower alcohol, for example, methyl alcohol, with a hydrocarbon mixture comprising, as the major component thereof, an unsaturated hydrocarbon compound, for example, isobutylene; subjecting the resultant reaction product mixture comprising the resultant ether compound and non-reacted portions of the lower alcohol and hydrocarbon mixture to a distillation to provide a distillation vapor fraction comprising the non-reacted lower alcohol and hydrocarbon mixture and a liquid fraction comprising the resultant ether compound that is collected from the distillation step; selectively recovering the non-reacted lower alcohol from the distillation vapor fraction by using a specific aromatic polyimide asymmetric separating membrane; and returning the recovered non-reacted lower alcohol to the reaction step.
Namely, in the process of the present invention, the reaction product mixture obtained from the reaction step is subjected to a distillation procedure in a distillation column, for example, a multi-pass distillation column to provide a distillation vapor fraction comprising the non-reacted lower alcohol and hydrocarbon mixture, which is collected through a top outlet of the distillation column and a liquid fraction comprising the resultant ether compound, which is recovered through a bottom outlet of the distillation column. The collected distillation vapor fraction is subjected to a pervaporation procedure in which a specific asymmetric separating membrane is employed and the non-reacted lower alcohol is selectively recovered with a high degree of purity and high efficiency by permeating through the separating membrane. The recovered vapor fraction comprising, as the major component thereof, the non-reacted lower alcohol is liquefied by cooling and then returned to the reaction step, to recyclically use the lower alcohol.
2. Description of the Related Art
It is known that an ether compound, for example, methyl-tert-butyl-ether, is produced by reacting a C.sub.4 petroleum fraction (hydrocarbon mixture) containing an unsaturated hydrocarbon compound, for example, isobutylene, with an excess amount of a lower alcohol, for example, methyl alcohol, as disclosed in U.S. Pat. Nos. 4,302,298, 4,447,653, 4,544,776, 4,605,787 and 4,774,365.
In this conventional process, the resultant reaction product mixture comprises the resultant reaction product, namely an ether compound, a non-reacted portion of the lower alcohol and a non-reacted portion of the unsaturated hydrocarbon compound.
A conventional process for recovering and refining the resultant ether compound and the non-reacted reactant compounds comprises a complicated combination of one or more distillation steps and one or more extraction steps. For example, the conventional process is carried out by employing a reaction column in which the lower alcohol is reacted with the hydrocarbon mixture, a distillation column in which non-reacted compounds are separated from each other and recovered, a water-washing column in which the distillation vapor fraction generated in the distillation column comprising the non-reacted lower alcohol is washed with water to extract the non-reacted lower alcohol in water, and a dehydration column in which the non-reacted lower alcohol is recovered by removing water from the lower alcohol-containing water extract.
The reaction of the lower alcohol with the unsaturated hydrocarbon compound is carried out in accordance with an equilibrium reaction theory, and therefore, the lower alcohol must be employed in an excessive amount to enhance the conversion of the unsaturated hydrocarbon compound. Therefore, after completion of the reaction, a large amount of non-reacted lower alcohol remains in the reaction product mixture. This non-reacted lower alcohol must be recovered from the resultant reaction product mixture.
When the recovery of the non-reacted lower alcohol is carried out by a conventional distillation procedure, an excessively large load needs to be applied to the recovery process and therefore the recovering (refining) apparatus becomes large and a large amount of energy for carrying out the recovering (refining) process is consumed.
AICHE Symposium Series, 85 (272), 82 (1989) discloses a process for producing methyl-tert-butylether, in which methyl alcohol is reacted with isobutylene to produce methyl-tert-butylether (MTBE); the resultant reaction product mixture is subjected to a pervaporation procedure using a separating membrane made of a cellulose acetate so as to recover a vapor fraction permeated through the separating membrane and comprising non-reacted portion of methyl alcohol, and a liquid fraction not permeated through the separating membrane; the permeated vapor fraction is returned to the reaction. step; the non-permeated liquid fraction is fed to a distillation-extraction process so as to separate the non-reacted isobutylene portion from the reaction product.
In the above-mentioned process, the cellulose acetate separating membrane exhibits a relatively low separation factor (CH.sub.3 OH/MTBE) of 5 to 20 and a relatively low separation factor (CH.sub.3 OH/C.sub.4 -fraction) of 1 to 3. Accordingly, it is difficult to recover the non-reacted methyl alcohol with a high degree of purity. Also, the non-permeated liquid fraction collected in a feed side of the separating membrane contains a certain amount of non-reacted methyl alcohol together with the resultant MTBE and the non-reacted isobutylene. Accordingly, it is difficult to recover the non-reacted methyl alcohol with high efficiency, unless the non-permeated liquid fraction is subjected to a distillation process, water-washing and dehydrating procedures similar to those of the above-mentioned conventional process.
Accordingly, the above-mentioned conventional processes are not satisfactory for practical use.